For Apollo-Soyuz Test Project, when an Apollo Command Module would dock with a Soyuz spacecraft, it was necessary to address the fact that the two types of spacecraft used different types of docking hardware; Apollo used one type of probe and drogue system and the Soyuz used another). The Soviets proposed the creation of an "international" docking mechanism and fitted such a docking mechanism to the Soyuz spacecraft to be used for ASTP.

Additionally, the two spacecraft used different atmospheres in their spacecraft: Apollo used a 100% oxygen environment at 5 psi and Soyuz used a mixed nitrogen-oxygen atmosphere (80-20) at about 14.7 psi.

To address both of these issues, the US developed a Docking Module which had an Apollo-type docking system on one end and an international docking mechanism on the other. The Docking Module also contained gas supplies and could serve as an airlock for moving from one spacecraft to the other.

Two docking modules were manufactured, the flight article and a backup, which is on display at the National Air & Space Museum.

Early in the ASTP program, a two-fifths scale engineering model of the DM was created for technical discussion concerning the mission.

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NASA photo 72-H-897.
From an eBay auction. Collection and clean-up by heroicrelics.org.

FOR RELEASE: Filed: July 19, 1972
PHOTO NO. 72-H-897

MANNED SPACECRAFT CENTER, HOUSTON, TEXAS

SOVIET/U.S. SPACE EXPERTS -- Soviet and American experts examine a two-fifths scale model of the docking mechanism being considered for use during the Apollo-Soyuz Test Project, a cooperative rendezvous and docking mission planned for 1975. A 22-member delegation from the Soviet Union is at the Manned Spacecraft Center (MSC) for technical discussions concerning the mission. Prov. Konstantin D. Bushayev, project director for the U.S.S.R., is in the left foreground. Academician Boris N. Petrov, Chairman of the Soviet Intercosmos Council and leader of the Soviet delegation here, is bending over examining the model. Next to Petrov is Glynn S. Lunny, project director for the U.S.

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Obverse of NASA photo 72-H-897.
From an eBay auction. Collection and clean-up by heroicrelics.org.

This engineering model, or one very similar to it, is displayed at Kennedy Space Center.